Epilepsy is a prototypical neural circuit disorder with a one-year prevalence of ~7/1,000 and high cost to society. Genetic or acquired etiologies of epilepsy lead to neuronal circuit hyper-synchrony that manifests as a seizure. A major unsolved question in epilepsy is how single units get recruited, in vivo, during the evolution of seizure events. Specifically it is not known whether neurons fire in a stereotyped pattern or sequence per seizure event, whether this happens reliably or whether/how it depends on cell type. It is important to determine whether pyramidal neurons show different patterns of recruitment than various classes of interneurons, and whether there exist special (hub) units that are reliably engaged and may therefore play an important role in recruiting other units to seizure events. The stargazer mouse is a validated experimental model for human absence epilepsy. Mutation of the protein stargazin leads to impaired AMPA receptor membrane trafficking, and this is thought to suppress primarily excitatory inputs projecting on inhibitory (Parvalbumin+) interneurons (Maheshwari et al., Frontiers in Cellular Neuroscience, 2013). This relative silencing of inhibition is thought to disinhibit the surrounding microcircuit, promoting hyper-synchrony and seizures. Whether this happens in vivo and how it entrains neocortical circuits remains unknown. The role that other interneuronal classes play remains also obscure. We will use chronic two-photon imaging to map how individual cortical neurons are recruited in vivo during stargazer absence seizure events, and to measure their temporal activity profiles and reliability of recruitment. Preliminary data suggests that recruitment is not random, but potentially depends on cell type, laminar location, and the neuron's hub status. Identifying groups of cells that exhibit high levels of synchrony will reveal local sub-networks important for seizure manifestation. Finally, in vivo whole-cell patch clamp experiments will be performed to 1) validate the two photon results, and 2) to study how inhibitory and excitatory inputs evolve during absence seizure events in pyramidal neurons versus in select classes of GABA-ergic interneurons. Optogenetic manipulation of Parvalbumin+ interneuron activity levels will establish causality. Obtained insights into epileptic circuit malfunction will potentially lead to new strategies for cell-targeted therapeutic interventions.